Karenia brevis (formerly known as Gymnodinium breve and Ptychodiscus brevis) is a marine dinoflagellate common in Gulf of Mexico waters, and is the organism responsible for Florida red tide, as well as red tide in Texas.[1]

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K. brevis is a microscopic, single-celled, photosynthetic organism that can "bloom" (see algal bloom) frequently along Florida coastal waters. Each cell has two flagella that allow it to move through the water in a spinning motion. K. brevis naturally produces a suite of potent neurotoxins collectively called brevetoxins, which cause gastrointestinal and neurological problems in other organisms and are responsible for large die-offs of marine organisms and seabirds.[2]K. brevis is unarmored, and does not contain peridinin. Cells are between 20 and 40 μm in diameter.

In its normal environment, K. brevis will move in the direction of greater light[3] and against the direction of gravity,[4] which will tend to keep the organism at the surface of whatever body of water it is suspended within. Cells are thought to require photosynthesis to obtain nutrition.[5] Its swimming speed is about one metre per hour.[6]K. brevis is the causative agent of Red Tide, when K. brevis has grown to very high concentrations and the water can take on a reddish or pinkish coloration. The region around southwest Florida is one of the major hotspots for red tide blooms. Red Tide outbreaks have been known to occur since the Spanish explorers of the 15th century, although not nearly as common, or for as lengthy a duration as now.[7] Some sources say Florida red tide blooms are about 10- to 15-fold more abundant than they were 50 years ago.[8] Algal species that have harmful effects on either the environment or human health are commonly known as Harmful Algal Blooms (HABs). HABs are harmful to organisms that share the same habitat as them, though only when in high concentrations.[2]

Traditional methods for the detection of K. brevis are based on microscopy or pigment analysis. They are time-consuming and typically require a skilled microscopist for identification.[9] Cultivation based identification is extremely difficult and can take several months. A molecular, real-time PCR-based approach for sensitive and accurate detection of K. brevis cells in marine environments has therefore been developed.[10] Another upcoming technique for the detection of K. brevis is multiwavelength spectroscopy, which uses a model-based examination of UV-vis spectra.[11] This particular protist is known to be harmful to humans, large fish, and other marine mammals. It has been found that the survival of scleractinian coral is negatively affected by brevetoxin. Scleractinian coral exhibits decreased rates of respiration when there is a high concentration of K. brevis.[2]